1. Technical Field
The present invention is directed to a DC-to-DC converter and is particularly directed to an apparatus for extending the Vout/Vin ratio of a commercially available DC-to-DC converter beyond the ratio limit specified by the original equipment manufacture.
2. Background Art
DC-to-DC converters are commercially available from several manufacturers. Such converters have a wide variety of applications in various arts and are particularly useful in the automotive art to generate a DC voltage level substantially greater than the voltage level provided by the vehicle battery. Not only does the converter provide an increased voltage level, but also regulates the converted voltage at that level.
One particular application of DC-to-DC converters in the automotive art is in the area of air bag diagnostic circuits. Such circuits use DC voltage levels substantially greater than the voltage level provided by the vehicle battery to perform specific diagnostic tests on system components. Some commercially available DC-to-DC converters have not proven adequate to provide and maintain a sufficient DC voltage level over the expected range of voltage levels provided by the vehicle battery. It is therefore desirable to extend the Vout/Vin ratio of a commercially available DC-to-DC converter beyond that specified by the manufacturer for such automotive applications.
Motorola Semiconductor Products Inc. manufactures a monolithic switching regulator subsystem intended for use as a DC-to-DC converter. This converter is commercially available under part number MC34063 ("the '063 converter"). The '063 converter includes an input for connection of a first DC voltage level, defined as Vin. An inductor is connected to the first DC voltage level through a current limiting resistor. A diode rectifier has its anode connected to the other side of the inductor. A filter capacitor is connected to the cathode of the diode rectifier and to electrical ground. The charge across the filter capacitor is defined as Vout.
The converter further includes an oscillator having a current source and a current sink. A timing capacitor is connected to the oscillator's current source and current sink. The oscillator outputs a control signal which is used to control actuation of a first switching device. The first switching device is connected to the junction of the inductor and diode rectifier and to electrical ground. The charging rate and the discharging rate of the timing capacitor respectively controls the ON and OFF time of the outputted oscillator control signal and, in turn, the ON and OFF time of the switching device.
When the switching device is ON, energy is stored in the inductor. When the switching device is switched OFF, energy, in the form of a voltage level, stored in the inductor is added to the Vin voltage value. The resultant output voltage after rectification and filtering, i.e., Vout, is greater than the Vin voltage. The Vout voltage value is a function of the ON vs. OFF time of the switching device, which is, in turn, a function of the ON vs. OFF time of the control signal outputted from the oscillator. The ON vs. OFF time of the oscillator control signal is functionally related to the charging rate and the discharging rate of the timing capacitor which is controlled by the oscillator's current source rating and the current sink rating. The manufacturer's specification for the '063 device limits the ON/(ON + OFF) ratio of the switching device to 0.857 for a typical current source rating of 35 microamps and a typical current sink rating of 200 microamps. This limits the Vout/Vin ratio to 6:1.
A Motorola application note AN920A suggests a circuit for extending the ON/(ON + OFF) ratio of the switching device to increase the Vout/Vin ratio. This suggested circuit includes a diode connected in series between the oscillator and the timing capacitor. The cathode of the diode connected to the timing capacitor. A PNP transistor has its base connected to the anode of the diode, its emitter connected to the cathode of the diode, and its collector connected to electrical ground. The suggested circuit does extend the operating ratio of the converter beyond that originally specified by the manufacturer. The particular arrangement, however, requires germanium components because of voltage values required for proper operation of the internal oscillator. As is well known in the art, germanium components are temperature sensitive and are expensive compared to silicon based devices.
Another problem with some commercially available DC-to-DC converters is that the converter's oscillator frequency is subject to substantial operating variations. This results from a loose tolerances of the oscillator's current source rating and current sink rating. To optimize core volume of the inductor, it is desirable to have a stabilized oscillator frequency at a known value.